Adhesion promoters are the paint additives that bond a paint film to its substrate, and formulators use 2 types: organofunctional silanes for glass, aluminum and mineral or metal surfaces, and chlorinated polyolefins (CPO) for polypropylene and other low-energy plastics. A silane couples the inorganic surface to the organic binder, which is why adhesion promoters are also called coupling agents.
Adhesion promoters are one of the 15 families of paint additives, and this page covers how each class bonds, selection by substrate, both substances, the defect they prevent, the rules, the tests and sourcing. APTES releases 0.624 g of ethanol per g on full hydrolysis, and a paint with 5 % or more of it is classified Skin Corr. 1. Aerosol adhesion promoters for car body repair are outside this page.
How we check: regulations and classifications are read at the body that issues them, standards at the standards body, and every additive level names its basis and its source; the full method is in methodology and fact-checking.
What Are Adhesion Promoters in Paint and Why Do Paints Need Them?#
An adhesion promoter is a paint additive or primer component that improves the bond between the paint film and its substrate. A silane bonds to the surface with one end and reacts with the binder with the other.
Paints need adhesion promoters on hydroxyl-rich glass and metal, on low-surface-energy plastics such as polypropylene and TPO (thermoplastic polyolefin), and where the film must survive water. APTES and GPTMS both improve wet adhesion, and GPTMS also improves water resistance.
Inside the Paint or as a Separate Primer Step?#
An adhesion promoter works either inside the paint as an additive or as a separate thin primer under it, and a chlorinated polyolefin gives its best overall adhesion as a separate primer. Additive use saves the priming step but is limited by compatibility. Eastman GN-408 sets its CPO primer at up to 5 wt% solids and 2.5-5 µm (0.1-0.2 mil) dry film thickness (DFT), topcoated after 2-3 min above 21 °C (70 °F).
The Gelest silane primer method adds silane to 2 % of a 95 % ethanol and 5 % water solution at pH 4.5-5.5, hydrolyzes it for 5 min, and cures the treated surface 5-10 min at 110 °C (230 °F) or 24 h at room temperature below 60 % relative humidity.
How Do Adhesion Promoters Bond Paint to a Substrate?#
Adhesion promoters bond paint in 2 ways: silanes form chemical bonds with both substrate and binder, and chlorinated polyolefins act at the interface between a low-surface-energy plastic and the paint. A silane, Y-(CH2)n-Si(OR)3, carries an organic group Y and methoxy or ethoxy groups, which release methanol or ethanol as they hydrolyze to silanols. The silanols condense with surface hydroxyl groups into oxane bonds, while an amine reacts with epoxy, isocyanate and acid groups of the binder and an epoxide with amines, acids and hydroxyls.
A borderline-compatible co-resin, such as an acrylic, acrylic-modified alkyd or polyester, can help a CPO reach the interface. Eastman GN-471 reports better adhesion when coated TPO parts are baked above 100 °C (212 °F).
2 Types of Adhesion Promoters for Paint Compared#
The 2 types of adhesion promoters for paint are organofunctional silanes, used on glass, aluminum and other hydroxyl-rich surfaces, and chlorinated polyolefins, used on polypropylene, TPO and polyethylene. The table compares chemistry, substrates, paint types, one sourced example level with its basis, strengths and limits.
| Class | Chemistry and members | Substrates | Paint types | Example level (product, basis, source) | Strengths | Limits |
|---|---|---|---|---|---|---|
| Silane adhesion promoters | Y-(CH2)n-Si(OR)3 with an amino, epoxy, methacryloxy, vinyl, mercapto or isocyanato group; methoxy or ethoxy; APTES, GPTMS | Glass, silica, aluminum (excellent); steel, zinc (slight); calcium carbonate, barytes (poor) | Epoxy, urethane and acrylic paints and primers on glass and metal | Gelest silane primer method: 2 % silane in 95 % ethanol / 5 % water at pH 4.5-5.5 (% of the primer solution, separate step); in-paint level: see the silane adhesion promoters page | Chemical bond to the substrate (oxane bond) and to the binder | Release methanol or ethanol on hydrolysis; weak on calcium carbonate, barytes and carbon black |
| Chlorinated polyolefin (CPO) adhesion promoters | Chlorinated polypropylene (CAS 68442-33-1), maleated chlorinated polypropylene (CAS 68609-36-9) | Polypropylene, TPO, polyethylene | Primers and paints for polyolefin plastic parts | Eastman GN-408: additive start 2.5 % CPO solids on unreduced paint; waterborne CPO primers at 8-10 % solids | Best overall adhesion as a separate primer; baking improves adhesion and gasoline resistance | Additive use limited by compatibility with the paint |
Each class page gives the members, grades and levels behind the example column.
How to Choose an Adhesion Promoter by Substrate and Paint Type#
Choose an adhesion promoter by the substrate first: a silane for glass, aluminum and other hydroxyl-rich surfaces, a chlorinated polyolefin for polypropylene, TPO or polyethylene, and a separate primer step where the best adhesion matters most. The table matches 3 paint types to class, example and watch-out.
| Paint type | Substrate | Class | Example (basis, source) | Watch-out |
|---|---|---|---|---|
| Primer formulation | Metal, glass, plastics | Silane or CPO by substrate | CPO primer up to 5 wt% solids at 2.5-5 µm (0.1-0.2 mil) DFT (Eastman GN-408) | EU Ecolabel (Decision (EU) 2025/2607): opaque primers with special adhesion properties need a spreading rate of 6 m²/L instead of 8 m²/L |
| Epoxy paint formulation | Glass, aluminum, steel | Amino or epoxy silane | See the epoxy paint formulation page | Methanol from trimethoxy silanes counts as VOC |
| Polyurethane paint formulation | Metal and glass | Amino silane (the amine reacts with isocyanate groups) | See the polyurethane paint formulation page | Ethanol from triethoxy silanes counts as VOC |
Primers carry the adhesion promoter among their critical additives, beside the anticorrosive pigment and the flash-rust inhibitor. Each paint type's full additive package is set out in paint formulations by paint type.
Waterborne paints narrow the choice: Gelest gives aqueous solutions of simple alkyl silanes a stability of 2-12 h, and Eastman GN-408 lists waterborne CPO primers at 8-10 % solids.
Which Substrates Respond to Silanes?#
Silanes bond excellently to silica, glass, aluminum and aluminosilicates, well to copper, tin, talc and iron, titanium and chromium oxides, slightly to steel, iron, nickel, zinc and lead, and poorly to calcium carbonate, gypsum, barytes, graphite and carbon black.
On metal, this ranking makes aluminum the substrate where a silane works best, while steel and zinc gain only slightly.
Why Low-Energy Plastics Need a Different Class#
Polypropylene, TPO and polyethylene need a chlorinated polyolefin, because silanes bond through hydroxyl groups on the surface, while CPO grades are made for low-surface-energy polyolefin plastics. The table lists CPO product examples by substrate and chlorine content.
| Product (producer, data sheet) | Use | Substrate or value |
|---|---|---|
| CP 153-2 (Eastman GN-408) | Primer | Polyethylene |
| CP 730-1, CP 343-1, CP 164-1 (Eastman GN-408) | Primer | Polypropylene, TPO, aluminum, galvanized steel |
| CP 343-3, CP 515-2 (Eastman GN-408) | Additive | Polypropylene, TPO, aluminum, galvanized steel |
| Hardlen solventborne grades (Toyobo product guide) | Primer or additive | 20-32 wt% chlorine |
| Hardlen waterborne grades (Toyobo product guide) | Waterborne paints | 16-17 wt% chlorine at 30 % solids; chlorine-free waterborne grades also offered |
Complete Table of Adhesion Promoter Substances Used in Paint#
The table lists both adhesion promoter substances with a page on this site, 3-aminopropyltriethoxysilane (APTES) and 3-glycidoxypropyltrimethoxysilane (GPTMS), with CAS and EC numbers, function, the alcohol each releases and the CLP hazard line.
| Substance | CAS, EC | Class | Function in paint | Alcohol on full hydrolysis | CLP hazard line |
|---|---|---|---|---|---|
| 3-Aminopropyltriethoxysilane (APTES) in paint | 919-30-2; 213-048-4 | Silane (primary amino, triethoxy) | Couples epoxy, urethane, phenolic and other binders to glass, aluminum, steel and mineral substrates; improves wet adhesion | 0.624 g ethanol per g (3 mol per mol) | Harmonized CLP Annex VI index 612-108-00-0: Acute Tox. 4* H302, Skin Corr. 1B H314 (GHS05, GHS07, Danger) |
| 3-Glycidoxypropyltrimethoxysilane (GPTMS) in paint | 2530-83-8; 219-784-2 | Silane (epoxy, trimethoxy) | Improves dry and wet adhesion and water resistance of epoxy, acrylic and urethane paints on inorganic substrates | 0.407 g methanol per g (3 mol per mol) | No Annex VI entry; notified (REACH joint submission): Eye Dam. 1 H318, Aquatic Chronic 3 H412 |
| All other paint additive substances |
Four further substances are sections of other pages: 3-aminopropyltrimethoxysilane (CAS 13822-56-5) on the APTES page, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (CAS 1760-24-3) on the silane page, and chlorinated and maleated chlorinated polypropylene on the CPO page.
Which Paint Defects Do Adhesion Promoters Prevent?#
Adhesion promoters prevent one paint defect, delamination and poor adhesion, in 3 forms: loss of adhesion on hydroxyl-rich mineral and metal surfaces, poor adhesion on polypropylene and TPO, and loss of wet adhesion. The table links each form to its fix.
| Defect form | Where | Adhesion promoter fix |
|---|---|---|
| Paint delamination and poor adhesion on glass, aluminum and mineral surfaces | Hydroxyl-rich substrates | Silane in the paint or as a primer solution |
| Poor adhesion on polypropylene and TPO | Low-surface-energy plastics | CPO primer or CPO additive |
| Loss of wet adhesion | Metal and glass | Amino silane |
Substrate wetting agents act on the same defect, helping the paint wet the surface it must hold to.
Every other defect, with the additive family that prevents it, is listed under paint defects.
How Are Adhesion Promoters Regulated? CLP, REACH and US Rules#
APTES is the only adhesion promoter substance on this page with a harmonized CLP classification: Annex VI index 612-108-00-0, Acute Tox. 4* H302 and Skin Corr. 1B H314, Danger. GPTMS has no Annex VI entry and is notified (REACH joint submission) as Eye Dam. 1 H318 and Aquatic Chronic 3 H412, and both CPO polymers are notified as not classified.
The table lists the instruments that reach adhesion promoter substances, with their dates.
| Instrument and date | What it requires | Adhesion promoter substances affected |
|---|---|---|
| CLP labeling of paints: Regulation (EC) No 1272/2008, Annex VI and Annex I generic limits | Mixture Skin Corr. 1 at 5 % or more of Skin Corr. 1 ingredients, Skin Irrit. 2 at 1 to below 5 %; Eye Dam. 1 at 3 % or more, Eye Irrit. 2 at 1 to below 3 % (Tables 3.2.3 and 3.3.3) | APTES (Annex VI entry); GPTMS and CPO (no Annex VI entry) |
| REACH registration, Candidate List and Annex XIV | APTES and GPTMS registered, not on the Candidate List (checked 2026-11-02), not in Annex XIV; CPO polymers exempt from registration (REACH Art. 2(9)), not on the Candidate List (checked 2026-10-27) | All 4 substances |
| Restricted and banned substances in paint: REACH Annex XVII entry 70 as replaced by Regulation (EU) 2024/1328, from 2026-06-06 | D4, D5 and D6 each below 0.1 % w/w; para 6(f) allows silicone-polymer residues up to 0.5 % for use as adhesion promoters | Adhesion promoters containing silicone polymers |
| VOC in paint: Directive 2004/42/EC Art. 2(5); 40 CFR 51.100(s) | EU: organic compound with an initial boiling point of 250 °C (482 °F) or less at 101.3 kPa; US: federal definition with its exempt list | APTES a VOC by boiling point; ethanol and methanol from hydrolysis count in both |
| EU Ecolabel for paints: Decision (EU) 2025/2607 | Adipic acid dihydrazide (CAS 1071-93-8) up to 1.0 % in the binder as an adhesion promoter or crosslinker | Adipic acid dihydrazide |
APTES boils at 217-220 °C (423-428 °F), below the 250 °C limit of Directive 2004/42/EC, so it is an EU VOC, and its full hydrolysis releases 0.624 g of ethanol per g. GPTMS boils at 290 °C (554 °F) and is no VOC by boiling point, but it releases 0.407 g of methanol per g, and neither alcohol is exempt under US 40 CFR 51.100(s).
In the US, all 4 substances are on the TSCA Inventory and 2024 CDR active. Rules for every family are on paint additive regulations. None is listed under California Proposition 65, but methanol, which GPTMS releases, is listed for developmental toxicity since 2012-03-16.
Since 2026-06-06, REACH Annex XVII entry 70 keeps D4, D5 and D6 each below 0.1 % w/w in any mixture placed on the EU market, and its paragraph 6(f) allows residues up to 0.5 % for use as adhesion promoters.
How Is Adhesion Promoter Performance Measured?#
Adhesion promoter performance is measured with 3 test groups: tape and cross-cut adhesion, pull-off strength, and knife or scrape adhesion, each with a current ASTM or ISO designation. The table assigns each group its standards.
| Test group | Standard and edition | What it shows for adhesion promoters |
|---|---|---|
| Paint adhesion testing: tape and cross-cut | ASTM D3359-23: Method A (X-cut) mainly for films above 125 µm (4.92 mils), Method B (lattice) for films up to 125 µm, ratings 5B to 0B; EN ISO 2409:2020: classes 0 to 5, not above 250 µm (9.84 mils) total thickness; ISO 16276-2:2025: on site, paint coatings on steel structures | Adhesion gain from a silane or CPO on glass, metal and plastics |
| Pull-off | ASTM D4541-22: portable testers, Protocol 1 to fracture, Protocol 2 pass/fail; ISO 4624 | Pull-off strength of the bond |
| Knife and scrape | ASTM D6677-18(2022) knife; ASTM D2197-16(2022) scrape on smooth flat panels | Qualitative lab and field check |
The 2 cross-cut scales run in opposite directions: ASTM 5B and ISO class 0 are both the best result, so 5B never equals ISO class 5. Every other test is described under paint test methods.
ASTM D3359 is similar to, but not technically equivalent to, ISO 2409.
Sourcing Adhesion Promoters: Product Forms, Specifications and Quote Requests#
Adhesion promoters are bought as neat liquid silanes and as chlorinated polyolefin grades, and a request names the class, the substance with CAS number or the grade, the substrate and paint type, annual volume, packaging and delivery country.
The Silquest A-1100 data sheet (Momentive) gives a flash point of 96 °C (205 °F) by PMCC and does not recommend ketones as diluents. The table lists current grades in alphabetical order of producer, without ranking.
| Substance | Grade (producer) |
|---|---|
| APTES (919-30-2) | Dynasylan AMEO (Evonik); Silquest A-1100 (Momentive); GENIOSIL GF 93 (Wacker) |
| GPTMS (2530-83-8) | XIAMETER OFS-6040 / DOWSIL Z-6040 (Dow); Dynasylan GLYMO (Evonik); Silquest A-187 (Momentive); GENIOSIL GF 80 (Wacker) |
Sourcing and qualifying producers is covered on paint additive suppliers and manufacturers.
Two related pages cover regional sourcing and the market.
- Producers and distributors in India: paint additive manufacturers in India
- Market size, segments and leading producers: paint additives market
How Adhesion Promoters Interact With Other Additive Families#
Adhesion promoters interact with 3 other paint additive families: wetting agents act on the same adhesion failure, corrosion inhibitors sit beside them in metal primers, and silicone defoamers fall under the same siloxane residue limits. The 3 are listed below.
- Wetting agents: the second additive fix for delamination; see wetting agents in paint.
- Corrosion inhibitors: a metal primer carries both families; see corrosion inhibitors for paint.
- Defoamers: the same D4, D5 and D6 limit applies; see paint defoamers.
Silanes and chlorinated polyolefins are also used in plastics and adhesives, uses this page does not cover.
How Much Adhesion Promoter Does a Paint Contain?#
Adhesion promoter levels are product values: Eastman GN-408 starts a CPO additive at 2.5 % CPO solids on unreduced paint and raises it until adhesion is obtained. Its example, 10 parts of CP 343-3 at 25 % nonvolatile per 100 parts of paint, supplies 2.5 parts of CPO solids (10 × 0.25).
A silane primer level is a share of the primer solution, not of the paint. Ladder studies that step a starting level up and down are explained under paint additive dosage.